Ceramic green sheet for porous layer, electrochemical element using the green sheet, and method of producing the element

ABSTRACT

A ceramic green sheet which is fired to give a porous ceramic structure, and which is formed of a composition which principally consists of a ceramic powder, a binder, and a multiplicity of planar or elongate flakes which disappear upon application of heat, and which are oriented in the ceramic green sheet in a direction substantially parallel to opposite major surfaces of the green sheet. Also disclosed is an electrochemical element having a protective layer covering at least one electrode, which layer is prepared from the ceramic green sheet indicated above, and has a multiplicity of planar or elongate pores corresponding to the planar or elongate flakes included in the green sheet. A method of producing the electrochemical element is also disclosed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to a ceramic green sheet, moreparticularly, to a ceramic green sheet which is used for providing anelectrode protective layer of an electrochemical element adapted todetect or determine the concentration of a given component of a subjectgas to be measured. The invention is also concerned with anelectrochemical element which has an electrode protective layer preparedfrom such a ceramic green sheet, and a method of producing such anelectrochemical element.

2. Discussion of the Prior Art

Porous ceramic structures or layers having a multiplicity of mutuallyconnected pores have been widely used for various purposes. For example,a porous ceramic layer is used as a gas-permeable protective layer whichcovers an electrode of an oxygen sensing element or otherelectrochemical element for determining the concentration of a givencomponent of a subject gas. This porous protective ceramic layer is usedfor protecting the electrode which is directly exposed to the subjectgas, e.g., an exhaust emission produced by internal combustion of anair-fuel mixture. Namely, the porous protective layer serves to protectthe electrode against adverse influences of corrosive substances, suchas lead, phosphorus, silicon and sulfur. When the electrode is exposedto these corrosive substances, the sensing element tends to suffer froma change in the control point, reduction in the output, anddeterioration of the operating response. The porous protective layeralso serves to prevent volatilization of the electrode components at anelevated temperature, thereby assuring improved durability of thesensing element. Where cracks occur in the protective layer during useof the sensing element, therefore, the electrode is directly exposed toa corrosive gas having a high temperature, resulting in corrosion andvolatilization of the exposed electrode. In this case, the sensingelement substantially loses its sensing capability.

The porous ceramic layer used as an electrode protective layer is formedby plasma-spraying a suitable ceramic material such as spinel so as tocover the electrode, and/or a substrate such as a solid electrolyte bodyon which the electrode is formed. Alternatively, a green sheet of asuitable ceramic material is laminated on a solid electrolyte body orother substrate, so as to cover the electrode formed on the substrate,and the green sheet is fired into the porous electrode protective layerintegral with the substrate. Generally, the porous ceramic layerprepared from a ceramic green sheet is given a suitable porosity, in oneof the following manners: (1) The green sheet is fired such that thesintering of the ceramic powder is insufficient; (2) The green sheetcontains a relatively large amount of binder; and (3) The green sheetcontains an organic additive or additives other than the binder, whichadditives are burned out during firing of the green sheet.

The porous ceramic layer formed by the plasma spraying technique has lowmechanical strength, and tends to suffer from cracking or flake-off dueto a thermal shock or stress. Further, it is generally difficult topractice the plasma-spraying technique, when the desired thickness ofthe porous ceramic layer exceeds 100 μm.

The porous ceramic layer formed by the green sheet lamination method hasa multiplicity of pores which provide relatively short fluid passages orchannels (gas-permeation paths) through which a gaseous fluid flows, forexample. In this case, the gaseous fluid flowing through the passagesimmediately reaches the surface of the substrate on which the porouslayer is formed. Accordingly, when the porous ceramic layer is used asan electrode protective layer of an electrochemical element, such as anoxygen sensing element, the protective layer is not able to sufficientlyremove corrosive substances contained in a subject gas. Consequently,the sensing element thus obtained tends to suffer from a change in thecontrol point, reduction in the output, and deterioration of theoperating response, and is therefore unsatisfactory in its durability.

SUMMARY OF THE INVENTION

The present invention was developed in view of the above-discussedbackground. It is accordingly a first object of the present invention toprovide a ceramic green sheet which is fired to give a porous ceramiclayer having comparatively long gas- or fluid-flow paths.

It is a second object of the invention to provide a highly durableelectrochemical element which has a porous ceramic structure obtainedfrom such a ceramic green sheet as described above.

It is a third object of the invention to provide a method of producingsuch a highly durable electrochemical element, in particular, an oxygensensing element.

The first object may be attained according to a first aspect of thepresent invention, which provides a ceramic green sheet which is firedto give a porous ceramic structure, and which is formed of a compositionwhich principally consists of a ceramic powder, a binder, and amultiplicity of planar or elongate flakes which disappear uponapplication of heat, and which are oriented in the ceramic green sheetin a direction substantially parallel to opposite major surfaces of thegreen sheet.

The ceramic green sheet prepared according to the present inventionincorporates the planar or elongate flakes which disappear uponapplication of heat, and which are oriented in the directionsubstantially parallel to the opposite major surfaces of the greensheet. Upon firing of the ceramic green sheet, the planar or elongateflakes as well as the binder are burned out, vaporized, sublimed, orthermally decomposed, to thereby provide generally planar or elongatepores corresponding to the planar or elongate flakes. Thus, there isobtained a porous ceramic structure or layer which includes amultiplicity of pores which are oriented in the direction substantiallyparallel to the major surfaces of the fired ceramic layer.

FIGS. 1(a) and 1(b) illustrate a porous ceramic sheet which is formed byfiring a ceramic green sheet prepared according to the presentinvention. As a result of vaporization, sublimation or thermaldecomposition of the planar or elongate flakes incorporated in theceramic green sheet, a multiplicity of pores 2 each having a generallyplanar or elongate shape are formed in the porous ceramic sheet, suchthat each pore extends in a direction substantially parallel to themajor surfaces of the ceramic sheet. These pores 2 communicate with eachother in the porous ceramic sheet, to thereby provide relatively longfluid passages or channels which extend from one of the opposite majorsurfaces of the porous ceramic sheet to the other surface, as indicatedby an arrow in FIG. 1(b).

On the other hand, a porous ceramic sheet which is obtained by firing aconventional ceramic green sheet has a multiplicity of amorphous pores4, as shown in FIGS. 2(a) and 2(b). As specifically shown in FIG. 2(b),the pores 4 are mutually connected to provide fluid passages or channelswhich extend from one major surface of the porous sheet to the othersurface, substantially in the direction of thickness of the ceramicsheet. Namely, the fluid permeation or flow paths formed through thisporous ceramic sheet do not have many portions which extend in adirection substantially parallel to the major surface of the ceramicsheet. Thus, the length of the fluid permeation paths formed through theconventional porous ceramic sheet is generally much smaller than that ofthe paths formed through the ceramic sheet as shown in FIGS. 1(a) and1(b).

The second object may be attained according to a second aspect of theinvention, which provides an electrochemical element comprising: (a) asolid electrolyte body; (b) a plurality of electrodes formed on thesolid electrolyte body; and (c) a protective layer covering at least oneof the electrodes. The protective layer consists of a ceramic porouslayer having a multiplicity of mutually connected pores each of whichhas a generally planar or elongate shape, the pores being oriented in adirection substantially parallel to the opposite major surfaces of theprotective layer.

In the electrochemical element constructed according to the presentinvention, the ceramic porous layer as the electrode protective layerhas generally planar or elongate pores which are oriented in thedirection substantially parallel to the major surfaces of the porouslayer. These pores communicate with each other to provide relativelylong fluid passages or channels, through which a subject gas to bemeasured flows toward the electrode that is covered by the protectivelayer. Accordingly, the electrode is prevented from direct exposure tothe subject gas, and is therefore protected against adverse influencesdue to corrosive substances contained in the subject gas. Consequently,the electrochemical element used as an oxygen sensing element, forexample, does not suffer from a change in the control point, reductionin the output, and deterioration of the operating response. Thus, thesensing element having the present porous protective layer hassignificantly improved durability.

The present electrochemical element may be easily and efficientlyproduced, since the porous protective layer covering the electrode maybe formed only by firing the ceramic green sheet of the invention.Further, the porous protective layer is formed integrally with theelectrode upon firing of the ceramic green sheet, thereby assuringimproved mechanical strength of the electrochemical element. Namely, theporous protective layer formed by firing the present ceramic green sheetis effectively protected from cracking due to a thermal shock, andseparation from the electrode, and can be formed with a desiredthickness.

BRIEF DESCRIPTION OF THE DRAWINGS

The above objects, features and advantages of the present invention willbe better understood by reading the following detailed description ofthe invention, when considered in connection with the accompanyingdrawings, in which:

FIGS. 1(a) and 1(b) are views showing an example of a porous ceramicsheet which is obtained by firing a ceramic green sheet of the presentinvention, FIG. 1(a) being a cross sectional view of the ceramic sheet,and FIG. 1(b) being a fragmentary enlarged view of FIG. 1(a);

FIGS. 2(a) and 2(b) are views showing an example of a porous ceramicsheet which is obtained by firing a conventional ceramic green sheet,FIG. 2(a) being a cross sectional view of the ceramic sheet, and FIG.2(b) being a fragmentary enlarged view of FIG. 2(a):

FIG. 3 is a fragmentary exploded perspective view showing an example ofan electrochemical element produced according to the present invention;

FIG. 4 is an elevational view in cross section taken along line 4--4 ofFIG. 3; and

FIGS. 5 and 6 are graphs indicating a result of a thermal shock test anda result of a Pb corrosion test, respectively, which were conducted onthe specimen according to the present invention and the comparativespecimens.

DETAILED DESCRIPTION OF THE INVENTION

The composition for the ceramic green sheet according to the presentinvention includes as a major component a ceramic powder which isselected from among conventionally used ceramic materials, dependingupon the application or use of the green sheet. The ceramic powder mayconsist essentially of alumina, spinel, cordierite or titania, or asolid solution of partially or fully stabilized zirconia with yttria,calcia or ytterbia. Alternatively, the ceramic powder may consistprincipally of such a solid solution, or a mixture of two or more of theceramic powder species specified above. To the ceramic powder, there maybe added a sintering aid or aids such as SiO₂, Al₂ O₃, kaolin and clay,as needed, in an amount of 30% by weight or less. In particular,alumina, and zirconia doped with yttria are preferably used as theceramic powder. More preferably, the ceramic powder consists principallyof partially stabilized zirconia whose major crystal phase is cubic, orfully stabilized zirconia having a cubic crystal phase.

The composition for the present ceramic green sheet includes a flakepowder in the form of planar or elongate (needle-like) flakes or flat orelongate particles as another important component, in addition to theceramic powder as described above. The planar or elongate flakessublime, thermally decompose or burn out, at a sintering temperature.Consequently, by firing the ceramic green sheet including such planar orelongate flakes, there is obtained a porous ceramic sheet or structurewhich has a multiplicity of mutually connected pores or channels.

The planar or elongate flakes may be selected from scaly or elongatepowder particles such as graphite and molybdenum disulfide, orflattened, planar or fibrous organic powder particles such as starch andsugar. According to the present invention, flake or crystalline powdersof graphite or molybdenum disulfide, for example, are preferably used asthe flakes to be contained in the ceramic green sheet. Generally, theplanar flakes are dimensioned such that the ratio of the length to thethickness thereof is at least 2/1 (2:1), preferably, at least 5/1 (5:1).It is to be understood that the opposite major surfaces of the planarflakes may have any shape, and may be more or less undulated or curled,and the opposite surfaces of each flake does not necessarily have thesame shape. Generally, the elongate flakes are dimensioned such that theratio of the length to the diameter thereof is at least 2/1(2:1),preferably, at least 5/1 (5:1). The planar or elongate flakes containedin the present ceramic green sheet may be of any kinds known in the art,provided the flakes have shapes considered to be planar or elongate, andhave dimensions satisfying the above conditions.

The composition of the ceramic green sheet according to the inventionincludes generally 20-80%, preferably 30-70% by volume of the planar orelongate flakes of a total amount of the ceramic powder and the planaror elongate flakes.

In addition to the ceramic powder and the planar or elongate flakes, thecomposition of the ceramic green sheet includes a binder as anotherimportant component, which serves to facilitate forming or molding ofthe composition into the green sheet. The binder may be suitablyselected from known binders which are conventionally used as a moldingaid for ceramic powder. For example, the binder is selected from thegroup including binders such as ethyl cellulose, polyvinyl alcohol,polyvinyl butyral, polyacrylate, and polymethacrylate, and othersynthetic resin binders. Depending upon the specific binders used, thecomposition may further include suitable plasticizers such as dioctylphthalate, dibutyl phthalate, dicthylene glycol and dibutyl sebacate.The binder and the plasticizer if any are generally added in a totalamount of 5-40 parts by weight, per a total of 100 parts by weight ofthe ceramic powder and planar or elongate flakes as described above.

When a slurry is prepared for producing the ceramic green sheet, thecomposition consisting principally of the ceramic powder, binder, andplanar or elongate flakes which have been described is mixed with asuitable amount of a solvent or a mixture of solvents, which is selecteddepending upon the specific binders and plasticizers used. Examples ofthe solvents suitably used for the slurry include: alcohol such asethanol, 2-propanol and 1-butanol; aromatic hydrocarbons such as tolueneand xylene; trichloroethylene; tetrachloroethylene; acetate; terpineol;carbitol; methyl ethyl ketone and water. The viscosity of the slurry isadjusted by the amount of the solvent.

By using the thus prepared slurry, a ceramic green sheet or layer havinga desired thickness is formed on a substrate, by a suitable known methodsuch as a doctor blade method, a calender roll method, screen printing,dipping or coating. According to the above-indicated methods, the slurryfor the ceramic green sheet is applied to the substrate while agravitational force acts on the applied slurry mass in the directionparallel to the major surfaces of the green sheet to be obtained. As aresult, the planar or elongate flakes contained in the applied slurrymass are oriented in the direction of the force being applied, so thatthese flakes are disposed in the resulting ceramic green sheet so as toextend substantially in the direction parallel to the major surfaces ofthe sheet. For more positively achieving such orientation of the planaror elongate flakes in the ceramic green sheet, a mechanical or physicalforce may be applied to the applied slurry mass which has been formedinto a sheet, in a direction parallel to the major surfaces of thesheet, while the slurry is still in a semiliquid state. In this respect,it is desirable to employ the doctor blade, calender roll or screenprinting method for achieving better orientation of the planar orelongate flakes in the ceramic green sheet obtained. The thus formedceramic green sheet has a thickness generally within a range of about20-1000 μm, preferably within a range of about 100-500 μm.

The ceramic green sheet formed on the substrate as described above isdried in air at the room temperature, or positively dried by heating, asneeded. The dried green sheet is then fired in the atmosphere, or in anoxidizing, reducing or inert atmosphere. In this manner, there isobtained a porous ceramic layer or sheet as shown in FIGS. 1(a) and1(b), which has planar or elongate pores that are oriented in thedirection parallel to the major surfaces of the ceramic body, and aremutually connected for communication therebetween. The thus obtainedporous ceramic body has a porosity generally in a range of about 20-80%,preferably in a range of about 30-60%.

The thus obtained porous ceramic sheet may be favorably used for variouspurposes, by utilizing its unique porous structure wherein the planar orelongate pores are oriented in the direction parallel to the majorsurfaces of the ceramic sheet. For instance, the present ceramic bodymay be used as a ceramic filter, or a partition plate or wall used on afuel cell, or may be suitably used as a protective layer for protectingan electrode or electrodes of an electrochemical element such as anoxygen sensing element, as will be described below.

Referring next to FIGS. 3 and 4, there is shown an example of the basicconstruction of such an electrochemical element in the form of an oxygensensing element having a porous electrode protective layer.

In these figures, reference numeral 10 designates an oxygen-ionconductive solid electrolyte body. On the opposite major surfaces of thesolid electrolyte body 10, there are formed a measuring electrode 12 anda reference electrode 14. On the major surface of the solid electrolytebody 10 which bears the reference electrode 14, there are formed aspacer layer 18 and a covering layer 20, such that the spacer layer 18is interposed between the solid electrolyte body 10 and the coveringlayer 20. The spacer layer 18 has an elongate rectangular opening, andcooperates with the solid electrolyte body 10 and the covering layer 20to define a gas inlet passage 16 for communication of the referenceelectrode 14 with a reference gas such as the ambient air. The majorsurface of the solid electrolyte body 10 which bears the measuringelectrode 12 is covered by a porous electrode protective layer 22 whichis prepared from a ceramic green sheet produced according to theprinciple of the present invention, and which has a porous structure asshown in FIGS. 1(a) and 1(b). In this arrangement, the measuringelectrode 12 is protected by the porous protective layer 22 from directexposure to a subject gas (measurement gas). Thus, the oxygen sensingelement has a laminar structure. It is to be understood that in thisoxygen sensing element, the solid electrolyte body 10, measuring andreference electrodes 12, 14, and spacer and covering layers 18, 20 areall made of conventionally used materials known to those skilled in theart.

In operation of the thus constructed oxygen sensing element, ameasurement gas such as exhaust gases emitted from an internalcombustion engine is brought into contact with the measuring electrode12 through the electrode protective layer 22, while the referenceelectrode 14 is exposed to the reference gas (e.g., an ambient air)which has a known oxygen concentration. An electromotive force isinduced between the measuring and references electrodes 12, 14,according to the principle of an oxygen concentration cell, based on adifference in the oxygen concentration between the atmospheres whichcontact or communicate with the two electrodes 12, 14. Thiselectromotive force is used as an output signal of the sensing element,which represents the oxygen concentration of the measurement gas.

In the instant oxygen sensing element, the protective layer 22 forprotecting the measuring electrode 12 exposed to the measurement gasconsists of a porous ceramic sheet having the porous structure as shownin FIGS. 1(a) and 1(b). In this case, the measurement gas passes throughrelatively long minute channels formed through the protective layer 22,such that the corrosive substances existing in the measurement gasflowing through the channels are prevented from reaching the measurementelectrode 12, due to absorption or deposition on the wall surfaces whichdefine the minute channels. Therefore, the measuring electrode 12 isprotected against deterioration by the corrosive substances in themeasurement gas, whereby the oxygen sensing element does not suffer froma change in the control point, reduction in the output, anddeterioration of the operating response. Thus, the instant oxygensensing element has a high degree of durability.

There will be hereinafter described a manner of producing the instantoxygen sensing element. Initially, there is prepared an unfired solidelectrolyte body (10) formed of a known solid electrolyte material andhaving a thickness of about 100 μm-1 mm. An unfired material for theelectrodes is applied to the unfired solid electrolyte body, by a knownmethod such as screen printing, transferring, spraying, coating orspinning, to thereby form unfired electrode layers (12, 14) having asuitable thickness (about 3-30 μm) on the solid electrolyte body (10).The material for the electrodes 12, 14 is well known in the art, forexample, an electrically conductive metal such as platinum, palladium orrhodium, or a cermet-forming mixture of such electrically conductivemetal and a ceramic powder such as alumina and zirconia. Then, anunfired material for the electrode protective layer 22, that is, aceramic green sheet according to the invention is formed on one of theopposite major surfaces of the unfired solid electrolyte body (10), bythe above-indicated known method. In this embodiment, the ceramic greensheet is formed so as to cover at least the unfired measuring electrode(12). Further, unfired masses for the spaced layer 18 and the coveringlayer 20 are formed on the other major surface of the unfired solidelectrolyte body (10), by using a known material which is usuallysimilar to the material of the solid electrolyte body 10. The unfiredlayers for the solid electrolyte body 10, electrodes 12, 14, electrodeprotective layer 22, and spacer and covering layers 18, 20 are co-firedto produce a fired laminar structure as the oxygen sensing element.

The same oxygen sensing element may be produced in an alternative manneras described below. Namely, a plurality of unfired electrode layers (12,14) are formed on the opposite major surfaces of an unfired solidelectrolyte body (10), and unfired spacer and covering layers (18, 20)are formed on the other major surface of the unfired solid electrolytebody if necessary, to thereby obtain an unfired laminar structure (10,12, 14, 18, 20). After firing of this laminar structure, the ceramicgreen sheet for the electrode protective layer 22 is formed so as tocover at least one (12) of the fired electrodes, by the above-indicatedmethod, and is fired to provide the electrode protective layer 22. In astill further alternative method of producing the oxygen sensingelement, the unfired layers for the reference electrode 14, and thespacer and covering layers 18, 20 if necessary, are formed on theunfired solid electrolyte body (10). After an unfired laminar structureobtained is then fired, the unfired layer for the measuring electrode 12and the ceramic green sheet for the protective layer (12) are formed onthe fired structure, and the unfired electrode layer (12) and the greensheet (22) are fired.

The oxygen sensing element may also be produced by using the solidelectrolyte body 10 which has been fired. In this case, the unfiredelectrode layers (12, 14) are formed on the fired solid electrolyte body10, and the ceramic green sheet for the porous protective layer isformed so as to cover the unfired measuring electrode (12). Further, theunfired spacer layer (18) and the unfired covering layer (20) are formedon the fired solid electrolyte body 10. Subsequently, the unfired layers(12, 14, 18, 20, 22) are co-fired to produce the oxygen sensing element.In the present method, sputtering, electroless plating, or vacuum vapordeposition as well as the above-indicated methods may be used forforming the unfired electrode layers (12, 14) on the fired solidelectrolyte body 10. When the sputtering, electroless plating or vacuumvapor deposition is practiced, the unfired electrodes (12, 14) may beformed with a thickness of about 0.3-5 μm.

For improving the integrity between the electrode protective layer 22and the solid electrolyte body 10, it is desirable that the coefficientof thermal expansion of the electrode protective layer 22 provided inthe present oxygen sensing element be substantially the same as or closeto that of the solid electrolyte body 10 which serves as a substrate forthe layer 22. To this end, the ceramic powder for the ceramic greensheet to give the protective layer 22 is preferably the same as thematerial for the solid electrolyte body 10. More preferably, theelectrode protective layer 22 is formed of a fully stabilized zirconiawhile the solid electrolyte body 10 is formed of a partially stabilizedzirconia. In this case the thermal stability of the protective layer 22and the strength of the solid electrolyte body 10 are compatible and areboth improved.

According to the present invention, the porous electrode protectivelayer 22 includes planar or elongate pores which occur upon sublimationof the planar or elongate flakes contained in the ceramic green sheet.In this regard, it is preferable that the protective layer 22 is formedof a composition which has a lower degree of sinterability than thematerial of the solid electrolyte body 10. In this case, the porousstructure of the layer 22 is provided with smaller or more minute poresin addition to the planar or elongate pores, when the protective layer22 and the solid electrolyte body 10 are co-fired.

The protective layer 22 may consist of a plurality of sub-layers, ratherthan a single layer. In this case, the sub-layer which is nearest to theelectrode 12 may have a higher porosity than the other sub-layer(s).Alternatively, the sub-layer remote from the electrode 12 may have ahigher porosity than the other sub-layer(s). Further, a second porouslayer whose porosity and pore size are different from those of theprotective layer 22 may be interposed between the layer 22 and themeasuring electrode 12.

While the present invention has been described as applied to the oxygensensing element as an electrochemical element having a typical basicconstructional arrangement, it is to be understood that the inventionmay be equally applied to the other types of known oxygen sensingelements having different constructional arrangements, and the otherelectrochemical elements.

For example, the electrochemical element using the ceramic green sheetof the present invention may be provided with suitable heating means formaintaining an optimum operating temperature of the element. In thisinstance, the heating means may be either incorporated or built in theelectrochemical element, or may be a separate heating member attached tothe electrochemical element.

Although the illustrated electrochemical element of FIGS. 3 and 4 has agenerally elongate planar configuration, the present invention may applyto an electrochemical element which has different configurations orshapes such as a tubular or cylindrical shape well known in the art. Theillustrated oxygen sensing element has only a single electrochemicalcell consisting of the solid electrolyte body 10 and the two electrodes12, 14. However, the invention is applicable to an electrochemicalelement having a plurality of electrochemical cells, for example, anelectrochemical oxygen pumping cell and an electrochemical oxygensensing cell, as disclosed in U.S. Pat. No. 4,861,456 corresponding tolaid-open Publication No. 60-108745 of unexamined Japanese PatentApplication No. 58-218,399.

EXAMPLES

To further clarify the principle of the present invention, there will bedescribed some presently preferred examples of the electrochemicalelement produced according to the invention. However, it is to beunderstood that the invention is by no means limited to the details ofthe examples, but may be embodied with various changes, modificationsand improvements, which may occur to those skilled in the art.

EXAMPLE 1

There was prepared a powder of zirconia (ZrO₂) whose purity is 99.5% byweight, and about 90% of which has a particle size not larger than 2.5μm. The ZrO₂ powder was wet-mixed with an aqueous solution of yttriumnitrate (85 g of Y₂ O₃ per 1 kg of the solution), in a pot mill by usingZrO₂ balls, for an hour, to provide a mixture which consists of 93 mol %of ZrO₂ and 7 mol % of Y₂ O₃. Then, the mixture was dried, and wascalcined at 1000° C. for two hours and thus calcined. The calcinedmixture was crushed into particles having 24 mesh size, to which 1% byweight of clay (24 mesh) was added. The thus obtained mixture wasdry-milled in a pot mill for 24 hours and passed through a 60-meshsieve, to thereby provide a ZrO₂ powder.

There was also prepared a crystalline graphite powder in the form ofscaly flakes, whose purity is at least 99.5% by weight, and about 90% ofwhich has a particle size not larger than 20 μm. In a pot mill usingzirconia balls having a diameter of 10 mm, there were introduced 70parts by weight of the ZrO₂ powder, 30 parts by weight of thecrystalline graphite powder, 10 parts by weight of polyvinyl butyralresin, 6 parts by weight of dibutyl phthalate, and 100 parts by weightof toluene+2-propanol mixture solvent. The solvent consists of 1 part byweight of toluene and 1 part by weight of 2-propanol. The introducedmaterials were mixed in the mill for ten hours, and the mixture waspassed through a 140-mesh sieve, whereby a slurry having a viscosity of10000 cps was prepared.

By using the obtained slurry, a ZrO₂ green sheet (ceramic green sheet)according to the invention was formed by a doctor blade method, so thatthe sheet has a thickness of 300 μm after the drying. Then, the greensheet was dried at 100° C. for two hours, and fired at 1400° C. for twohours, whereby a porous ceramic sheet was prepared. The ceramic sheethas a multiplicity of pores which are orientated in a direction parallelto the major surfaces of the sheet, as shown in FIGS. 1(a) and 1(b), andhas an open pore percent of 50%.

EXAMPLE 2

There was prepared a powder of Al₂ O₃ whose purity is 99.9% by weight,and about 90% of which has a particle size not larger than 2 μm. As asintering aid, 3% by weight of kaolin was added to the Al₂ O₃ powder,and the obtained mixture was dry-milled in a pot mill for 24 hours, byusing alumina balls having diameter of 10 mm, and passed through a60-mesh sieve, to provide an Al₂ O₃ powder.

There was also prepared a powder of molybdenum disulfide (MoS₂) in theform of scaly flakes, whose purity is at least 99% by weight, and about90% of which has a particle size not larger than 10 μm. In a pot mill byusing alumina balls having a diameter of 10 mm, there were introduced 50parts by weight of the Al₂ O₃ powder, 50 parts by weight of the MoS₂powder, 10 parts by weight of polybutyl methacrylate, 2 parts of dioctylphthalate, and 50 parts of toluene as a solvent. The introducedmaterials were mixed in the mill for ten hours, and the mixture waspassed through a 140-mesh sieve, whereby a slurry having a viscosity of20000 cps was prepared.

By using the obtained slurry, an alumina green sheet (ceramic greensheet) according to the invention was formed by a doctor blade method,so that the sheet has a thickness of 300 μm after the drying. The greensheet was fired under argon gas at 1200° C. for two hours, whereby aporous ceramic sheet was prepared. The ceramic sheet has a multiplicityof pores which are orientated in a direction parallel to the majorsurfaces of the sheet, as shown in FIGS. 1(a) and 1(b), and has an openpore percent of 40%.

EXAMPLE 3

A planar ceramic green sheet having a thickness of 500 μm was formed ofa composition which includes 100 parts by weight of a powder consistingof 96 mol % of ZrO₂, 4 mol % of Y₂ O₃, and 3% by weight of clay as asintering aid, 12 parts by weight of polyvinyl butyral resin, and 5parts by weight of dioctyl phthalate.

By using the obtained ZrO₂ green sheet as a solid electrolyte body 10,an oxygen sensing element as shown in FIGS. 3 and 4 was produced in thefollowing manner. A 10 μm thickness of electrically conductive paste foran electrode was applied to each of the opposite major surfaces of thegreen sheet, by screen printing. The electrically conductive pasteconsists of 80 parts by weight of a platinum powder, and 20 parts byweight of a ZrO₂ powder containing 4 mol % of Y₂ O₃. Then, theelectrically conductive paste was dried at 100° C. for 20 min., toprovide unfired electrode layers (12, 14) on the ZrO₂ green sheet.

Thereafter, a ceramic green sheet which gives the porous ceramic sheetas produced in Example 1 was formed on one major surface of theabove-indicated ZrO₂ green sheet so as to cover one of unfired electrodelayers (12), to provide an unfired electrode protective layer (22). Onthe other major surface of the ZrO₂ green sheet on which an unfiredelectrode layer (14) is formed, there were formed an unfired spacerlayer (18) and an unfired covering layer (20) both of which consist ofthe same ZrO₂ green sheet. The laminated unfired layers were compactedunder pressure and heat to provide an unfired laminar structure (10, 12,14, 18, 20, 22). This laminar structure was then fired in the atmosphereat 1400° C. for two hours. In this manner, there was produced the oxygensensing element (electrochemical element) of FIGS. 3 and 4 having theelectrode protective layer 22 in which planar pores are oriented in thedirection substantially parallel to the major surface of the element.The produced oxygen sensing element demonstrates a high degree ofdurability.

EXAMPLE 4

To the other major surface of the planar ZrO₂ green sheet for the solidelectrolyte body 10 as produced in Example 3, there was applied byscreen printing a 10 μm thickness of electrically conductive paste,which consists of 80 parts by weight of a platinum powder, and 20 partsby weight of a ZrO₂ powder containing 4 mol % of Y₂ O₃. Then, theapplied paste was dried at 100° C. for 20 min., to provide an unfiredreference electrode layer (14). On the other major surface of the ZrO₂green sheet, there were also formed an unfired spacer layer (18) and anunfired covering layer (20) as shown in FIGS. 3 and 4, to provide anunfired laminar structure (10, 14, 18, 20). This laminar structure wasthen fired at 1400° C. for two hours, in the atmosphere.

Subsequently, an unfired measuring electrode layer (12) made of platinumand having a thickness of 1 μm was formed on the above-indicated onemajor surface of the fired ZrO₂ green sheet (solid electrolyte body 10),by electroless plating. Then, a ceramic green sheet which gives theporous ceramic body as produced in Example 2 was laminated on theunfired electrode layer (12), and the thus laminated unfired layers werecompacted under pressure and heat to provide an unfired laminarstructure on the solid electrolyte body 10.

This laminar structure was fired at 1200° C. for two hours, under argongas, whereby there was produced the highly durable oxygen sensingelement having the measuring electrode 12 which is covered by the porousprotective layer 22 in which pores are oriented in a directionsubstantially parallel to the layer 22.

EXAMPLE 5

To the other major surfaces of the planar ZrO₂ green sheet for the solidelectrolyte body 10 as produced in Example 3, there was applied byscreen printing a mass of electrically conductive paste which consistsof 80 parts by weight of a platinum powder, and 20 parts by weight of aZrO₂ powder containing 4 mol % of Y₂ O₃. Then, the applied paste wasdried at 100° C. for 20 min., to provide an unfired reference electrodelayer (14). On the other surface of the ZrO₂ green sheet for the solidelectrolyte body 10, there were also formed unfired spacer and coveringlayers (18, 20) in the form ZrO₂ green sheets similar to the unfiredsolid electrolyte body (10). Thus, there was provided an unfired laminarstructure (10, 14, 18, 20). This laminar structure was then fired in theatmosphere at 1400° C. for two hours.

On the above-indicated one major surface of the fired solid electrolytebody 10, there was formed an unfired measuring electrode layer (12) madeof platinum and having a thickness of 0.7 μm, by high-frequencysputtering. Then, a mass of paste for forming the electrode protectivelayer 22 was applied by screen printing to the one major surface of thefired solid electrolyte body 10, to form a 100 μm thickness unfiredlayer (22) covering the measuring electrode layer (12). The unfiredlayers (12, 22) were dried at 150° C. for 30 min. under argon gas, andthen fired at 1200° C. Thus, there was produced the highly durableoxygen sensing element having the porous protective layer 22 asdescribed above.

The paste for forming the protective layer 22 was prepared in thefollowing manner. Initially, a ceramic powder was prepared from a ZrO₂powder containing 6 mol % of Y₂ O₃ (prepared by coprecipitation andhaving an average particle size of 0.3 μm), to which 3% by weight ofclay was added. The ZrO₂ powder with the clay was dry-milled for fourhours in a pot mill using ZrO₂ balls, and passed through a 60-meshsieve, to provide ZrO₂ particles. Then, 70 parts by weight of these ZrO₂particles were mixed with 30 parts by weight of a MoS₂ powder as scalyflakes (whose purity is 99% by weight, and about 90% of which has aparticle size not larger than 20 μm). Further, 12 parts by weight ofpolybutyl methacrylate and 100 parts by weight of terpineol were addedas a solvent to 10 parts by weight of the mixture of the ZrO₂ and MoS₂powders, and then mixed with the same for five hours in a pot mill, byusing ZrO₂ balls having a diameter of 10 mm. Thereafter, the viscosityof the obtained mixture was suitably adjusted to provide the paste forforming the protective layer 22, which is suited for use in the screenprinting method.

Evaluation of Oxygen Sensing Elements

The oxygen sensing element constructed as shown in FIGS. 3 and 4 wastested for evaluating the durability of the element. In the test, theoxygen sensing element produced according to Example 3 was used as aspecimen of the present invention. A comparative oxygen sensing element"A" was prepared by using an electrode protective layer (22) in the formof a 100 μm-thick spinel layer which are formed by plasma spraying. Inpreparing a comparative oxygen sensing element "B", an unfired electrodeprotective layer (22) was formed by co-firing unfired masses ofelectrodes and solid electrolyte body, and a green sheet which includesas a material for forming pores a multiplicity of amorphous particleswhich sublime upon application of heat.

(1) Thermal Shock Test

The test specimens were subjected to repeated heat-and-cool cycles byusing a burner. In each cycle, the specimens were heated to 1000° C. andcooled to 200° C. After the heat-and-cool cycles, the specimens wereobserved for occurrence of cracks in the electrode protective layer 22,and separation between the protective layer 22 and the measuringelectrode 12, and were evaluated in terms of reliability by usingWeibull distribution sheet.

(2) Pb Corrosion Test

Each of the test specimens was installed in a predetermined positionwithin an exhaust pipe of a 2.0 l gasoline engine. The engine wascontinuously operated with 0.5 gPb/gal leaded gasoline while emitting anexhaust gas having a temperature of 700° C. The excess air ratio (λ) ofthe exhaust gas was measured by the oxygen sensing elements of the testspecimens. More specifically, the excess air ratio of the exhaust gaswas extremely slowly changed (e.g., from 0.95 to 1.20), to measure theexcess air ratio value at the time when the electromotive forcegenerated by the sensing elements suddenly changes. An amount of changein the excess air ratio measured during the exposure of each specimen tothe exhaust gas was obtained. The corrosion resistance of the specimensagainst the lead (Pb) existing in the exhaust gas increases with adecrease in the amount of change in the measured excess air ratio.

The result of the thermal shock test is indicated in FIG. 5, while theresult of the Pb corrosion test is indicated in FIG. 6. It will beunderstood that the comparative oxygen sensing element "A" has anextremely low thermal shock resistance while the comparative oxygensensing element "B" has an extremely low corrosion resistance withrespect to lead. The instant oxygen sensing element is considerablyimproved in both the thermal shock resistance and the corrosionresistance, over the comparative elements "A" and "B". In the instantoxygen element, the electrode protective layer (22) consists of a porousceramic layer having a porous structure in which a multiplicity of poresare oriented in the direction parallel to the major surfaces of theelement, which pores cooperate to provide relatively long fluid flowchannels. Thus, the instant element is excellent in the adherencebetween the solid electrolyte body and the electrodes, assuring improvedcapability of trapping corrosive substances.

What is claimed is:
 1. A ceramic green sheet which is fired to give aporous ceramic structure, and which is formed of a composition whichconsists essentially of a ceramic powder, a binder, and a multiplicityof planar or elongate flakes, wherein said planar or elongate flakes areoriented in said ceramic green sheet in a direction substantiallyparallel to opposite major surfaces of the green sheet and decomposeupon application of heat to thereby provide the porous ceramic structurewith pores shaped and oriented substantially the same as said planar orelongate flakes.
 2. A ceramic green sheet according to claim 1, whereinsaid ceramic powder consists principally of partially or fullystabilized zirconia, alumina, spinel, cordierite or titania.
 3. Aceramic green sheet according to claim 1, wherein said planar orelongate flakes is selected from the group consisting of graphite,molybdenum disulfide, starch and sugar.
 4. A ceramic green sheetaccording to claim 1, wherein the ratio of a length to a thickness ofsaid planar or elongate flakes is at least 2/1.
 5. A ceramic green sheetaccording to claim 1, wherein the ratio of a length to a thickness ofsaid planar or elongate flakes is at least 5/1.
 6. A ceramic green sheetaccording to claim 1, wherein said composition includes 20-80% by volumeof said planar or elongate flakes of a total amount of said ceramicpowder and said planar or elongate flakes.
 7. A ceramic green sheetaccording to claim 1, wherein said composition includes 30-80% by volumeof said planar or elongate flakes of a total amount of said ceramicpowder and said planar or elongate flakes.
 8. A ceramic green sheetaccording to claim 1, further comprising a plasticizer, and wherein saidbinder and said plasticizer are present in a total amount of 5-40 partsby weight, per a total of 100 parts by weight of said ceramic powder andsaid planar or elongate flakes.
 9. A porous sintered ceramic articleconsisting essentially of:a first major surface and an opposed secondmajor; and a porous structure having a plurality of interconnected poreseach of which has a generally planar to elongate shape and is orientedin a direction substantially parallel to said opposed major surfaces,whereby said porous structure provides gaseous communication from saidfirst major surface to said second major surface.